The present invention relates generally to tear strips within a polymer material, particularly tear strips that are within and continuous with a polymer sheet having molecular chains that were reoriented during formation of the sheet. This invention encompasses an improved product having a tear strip, a method of producing such product, and an improved apparatus by which the product is formed. Tear strips are provided by developing molecular chain orientations along a preselected path which are different from molecular chain orientations in the rest of the sheet.
Heretofore, it has not been known to provide tear paths that are of substantially the same thickness and overall strength as the flexible sheet in which they are located and which are substantially continuous with that sheet. Packaging such as that shown in Rohde U.S. Pat. No. 3,186,628 forms groove lines within the thermoplastic flexible sheet materials by means of a probe projected into the path of the material as it is being extruded or by a heated bar in contact with the thermoplastic material, in either case thinning the material at the tear location. Since such groove lines reduce the thickness of the sheet therealong, they reduce the overall strength of the sheet and of items, such as packages or enclosures, made therewith. Also, when particular enclosures such as bag overpouches for containers of intravenous solution are made of these types of sheet materials, the thickness of the material throughout the enclosure can be important, inasmuch as rates of fluid passage through the material, such as water vapor transmission and gas permeability, will increase as material thickness decreases.
Representative of other approaches in this area are Rohde U.S. Pat. Nos. 3,368,740 and 3,535,409 which show tear lines being formed by coextruding two thermoplastic materials of different character, one of the extrusions being in the nature of a very narrow band which serves as a severance means. By this approach, there is formed a multiple-part sheet rather than the strong, continuous or single-part sheet according to the present invention, even though tear lines made in accordance with this prior approach are said to be able to be formed to the same thickness as the other parts of the sheet.
Forming a tear path either by reducing the thickness of the sheet thereat or by coextruding a separate tear strip insert with the sheet increases the possibility of developing leaks at the tear strips and of reducing the integrity of packages prepared therewith. It is therefore desirable to provide filming or sheeting having a tear path that is both substantially the same thickness as the rest of the sheet and is a continuous part thereof in order to thereby decrease the total, overall strength of the film or sheet to a very limited extent while at the same time provide a path for tearing the material at a predetermined location.
Features of this type are realized according to the present invention by fixing the generally axial molecular chain orientation pattern developed as the films or sheets are extruded and before the molecular chains are reoriented, while avoiding any significant change in the thickness of the sheet or film. In general, this is accomplished by rapidly extracting heat at a relatively small area near the location at which the sheet is extruded and before the location at which it is reoriented in order to form a tear path as the sheet is extruded.
It is accordingly a general object of the present invention to provide a sheet having an improved tear path therein.
Another object of the present invention is to provide a method, product, and apparatus that are improved in connection with a tear strip or path that is continuous with the rest of the film or sheet.
Another object of this invention is an improved method, product, and apparatus which provide an easy opening access path that has strength characteristics that are substantially uniform throughout the product and resists leakage when sterilized.
Another object of this invention is an improved enclosure product, and a method and means for producing an enclosure product, having a tear path that the unaided eye cannot distinguish from the rest of the enclosure unless the path is supplementarily marked.
Another object of the present invention is a method, apparatus, means, and product produced thereby which provide a polymer film or sheet having a path including polymer molecular chains oriented generally in-line with respect to each other, the in-line orientation being significantly different from the relative locations of the remaining film molecular chains with respect to each other.
Another object of the present invention is a method, means, apparatus and product produced thereby which include freezing or otherwise fixing a preselected pathway within a sheet of polymer material.